Semiconductive oxides

ABSTRACT

Semiconducting oxides of the formula Fe2 xGexO3 and shaped thermistor bodies comprising such oxides.

United States Patent Patterson June 17, 1975 SEMICONDUCTIVE OXIDES 3,515,686 6/1970 Bowman 252/518 41 197 2 [75] Inventor: Frank Knowles Patterson, 3560' 0 2/ I Schubert 25 618 Wilmington, Del. [73] A E I d P t d N & FOREIGN PATENTS OR APPLICATIONS ssigneez u on e emours Company, Wilmington, Del. 27555 8/1973 Japan [22] Filed: Oct. 15, 1973 Primary Examiner-Maynard R. Wilbur [211 App! 406303 Assistant Examiner-S. C. Suczinski [52] U.S. Cl. 252/519; 252/520; 423/633 [51] Int. Cl. H01b 1/08 [58] Field of Search 252/518, 519, 520; ABSTRACT semiconducting oxides of the formula Fe ,,Ge, O [56] References Cited and shaped thermistor bodies comprising such oxides.

UNITED STATES PATENTS 2.590.894 4/1952 Sanborn 252/520 8 Claims, 2 Drawing Figures g IIOOC LI) U f I00: N I

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WT. /o Ti 0 (BASED ON TOTAL Fe O TiO SEMICONDUCTIVE OXIDES BACKGROUND OF THE INVENTION This invention relates to novel metal oxides, and, more particularly, to semiconductive iron oxides useful as thermistors.

Thermistors are semiconductors exhibiting large variations of resistance with temperature, that is, a large temperature coefficient of resistance (TCR). When the resistance varies negatively with temperature, the thermistor is said to have a negative TCR; when the resistance varies positively with temperature, the thermistor is said to have a positive TCR. There exists a need for negative TCR thermistors and compositions for producing the same.

The applications for NTC (negative temperature coefficient) thermistors are principally temperature sensing, environmental sensing, current control and power. Most of the materials presently used for NTC thermistors are based on the principle of controlled valency of semiconducting oxides. It is thought that the condition for electrical conductivity in these semiconductors is that the lattice network contains ions of different valency on the same crystallographic site. This is achieved by the incorporation into the lattice of foreign ions of such a charge that they balance the charge of the ions of deviating valency.

Currently used thermistor materials include such semiconducting oxides as spinels, having 3 metal ions per four oxygen ions (Me O typically Ni, Cu- ,,*Mn,,* Mn O doped NiO such as Li Ni, Ni and oxides with the alpha-Fe O structure such as Fe Fe,, Me,, O (Me is Ti; see E. J. W. Verwey, Philips Res. Rep. 5, 173-187, at 183, 1950).

Fabrication of the aforementioned materials for thermistor applications generally involves admixing the appropriate reagents, which are then pressed to the desired configuration and thereafter sintered in a furnace at temperatures of 1200-1450C., to achieve compound formation and proper oxidation states.

In the firing step for the spinels and doped NiO, careful control of the furnace atmosphere is critical to maintain the ions in their proper oxidation state. Compositions based on the alpha-Fe O structure, e.g., Fe ,Ti,O although not as sensitive to the furnace atmosphere, require high temperatures (e.g., 1260C.) to complete reaction and achieve the proper resistivity; when these compositions are tired at lower temperatures, the reaction does not go to completion, and thus the resistivity of the composition can vary as a function of subsequent processing temperature to form shaped thermistors. Sanborn US. Pat. No. 2,590,894, issued Apr. 1, 1952, describes the Fe Ti,O compounds and their preparation; e.g., 2300F. or 1260C. was used at col. 2, line 30.

SUMMARY OF THE INVENTION I have invented a novel composition of matter useful as a thermistor. The novel composition of matter is semiconductive oxides of the hexagonal alpha-Fe O crystalline structure having the formula Fe ,Ge O x is in the range 0.0001-005. Optimum x for lowest resistivity is about 0.05, since this seems to be the maximum amount of GeO which can be inserted into the alpha-Fe O lattice. Stated another way, the novel oxide may be made by heating together 0.0l-l0% GeO and 9099.99% Fe O Geo would correspond to an x of 0.156 if reaction went that far), preferably 0.01-5% G20 and -99.99% Fe O The optimum weight percent of 0e0 for obtaining lowest resistivity, is about 3.24%, which corresponds to the maximum amount of GeO which can be inserted into the Fe O lattice (x 0.05). These oxides have a negative temperature coefficient of resistance, as described above.

These novel oxides offer the distinct advantage of having lower snythesis temperatures than some other doped iron oxides, such as Ti-doped iron oxide. They are also relatively insensitive over broader thermal processing range than are the Fe O /TiO systems.

Also a part of this invention are electrically conductive shaped ceramic bodies having a negative TCR (e.g., thermistors) of such novel oxides and such bodies which comprise, in addition to the novel semiconducting oxides Fe ,Ge O unreacted excess GeO in the latter case, the total GeO present, as unreacted GeO and/or as part of Fe Ge O is up to about 10 weight percent, based on the total weight of the Fe Ge O and GeO present. The 10% upper limitation is chosen for practical reasons, such as obtaining reproducible thermistors, it being understood that it may be possible to employ more than 10% GeO but still obtain thermistor behavior. Preferred bodies comprise up to 5% Geo on the same basis.

BRIEF DESCRIPTION OF THE DRAWING In the drawing,

FIG. 1 is a plot of sensitivity versus GeO content in Fe O /GeO systems of this invention formed at various temperatures, and

FIG. 2 is the same for Fe O /TiO systems, illustrating the remarkably greater temperature sensitivity of such oxides versus those of the present invention.

DETAILED DESCRIPTION Substitution of germanium ions into the crystal lattice of alpha-Fe o produces semiconducting oxides which are characterized by varying resistivities and NTCR characteristics, dependent upon the amount of GeO in the oxide. Similar characteristics can be achieved with TiO substitution in the alpha-Fe O lattice (as disclosed in US. Pat. No. 2,590,894); however, where GeO is used, lower synthesis temperatures can be used (e.g., 1000C.) and, more importantly, the resistivities of Ge-based compositions are relatively insensitive to a broader range of processing temperatures (1000-l200C.) than the corresponding Ti-based systems. This is shown in the comparative examples of this invention.

The oxides of the present invention may be made by firing the oxides Fe o and GeO in the desired proportions at 9501250C. in air. Often, the component oxides will be milled together, pressed into a body, and cosintered (coreacted) at such temperatures, to form a thermistor body.

The Figures illustrate the relative temperature insensitivity of the oxides of the present invention, a distinct commercial advantage. Iron oxide itself is reported to have a resistivity of 10 ohm-cm.

The crystalline structure of the oxides of the present invention was examined as follows. Incorporation of GeO into the hexagonal alpha-Fe o lattice by high temperature solid state synthesis techniques can be rationalized by the following molar equation:

The condition for electrical conductivity in semiconductors of this type is that the lattice network contain ions of different valency on the same crystallographic sites. That Ge does substitute on the octahedral sites of the hexagonal alpha-Fe O lattice, maintaining the same symmetry, is shown by x-ray powder diffraction on the reaction product of Fe O with 3.24% GeO (based on total weight of Fe O and GeO fired at llC. for 48 hours. A Hagg Gunier camera using CuKoz radiation was used. The powder pattern was indexed on the basis of the hexagonal alpha-Fe O lattice. All reflections could be accounted for. No GeO reflections were noted. The lattice parameters were refined by least squares techniques. The unit cell parameters of this compound are shown below and compared with the cell parameters (in angstroms) of alpha-Fe O reported under ASTM Card No. 13-534:

The following examples illustrate the present invention. In the examples all ratios, percentages, proportions, etc., are by weight.

Fe ,,.Ge ,.O was prepared by milling together for 18 hours lOO-gram quantities of commercially available reagent grade Fe O and GeO in the desired proportions, as an aqueous slurry; one-fourth inch long zirconia cylinders were used as the grinding medium. The resultant powder mixture was dried and then compressed into cylindrical rods (0.5 cm. diameter by 0.5 cm. length), which were placed on thick Pt foil and inserted into a furnace preheated to 800C. The temperature was then raised to the desired reaction temperature (either 1000C., 1 100C. or l200C. in three series of experiments) and held there for 48 hours. The resultant product was then quenched in air. For comparative purposes, Fe Ti 0;, was similarly prepared.

Table I indicates resistivity data for various oxides of this invention when formed at 1000C., llO0C. or l200C., respectively.

TABLE I RESISTIVITY FOR COMPOSITIONS OF Fe ,Ge,O;, WHEN PROCESSED AT VARIOUS TEMPERATURES Resistivity (ohm-cm.) at 25C. Measured on Samples Fired for TABLE I-Continued RESISTIVITY FOR COMPOSITIONS OF Fe ,Ge,O WHEN PROCESSED AT VARIOUS TEMPERATURES Resistivity (ohm-cm.) at 25C. Measured on Samples Fired for 48 Hours at Calculated* Value of Wt.

x" GeO 1000C. 1 100C. l200C.

It is thought that the maximum amount of GcO, which can be inserted in the Fe O, lattice in producing Fe Gc,0 corresponds to at equal to 0.05. Hence an x" above that value corresponds to a mixture of Fe ,Ge,O;, and unreacted GeO,.

"Based on total Fe O and Geo, present.

The resistivity data in Table l were calculated from resistance measurements as follows. A Viking LS 232 mercury-indium-thallium alloy obtainable from Victor King Materials Lab was applied uniformly on both faces of the fired cylindrical rods. Copper disc electrodes also coated with the same alloy were pressed onto the alloy-coated rods. Tinned copper leads were soldered onto the copper discs and connected to a Triplett type 1 digital volt ohmmeter, Model 8035. Resistance readings were taken at C. Resistivities were calculated in ohm-cm. using the equation:

rho'l R A where R resistance in ohms rho resistivity in ohm-cm. l length of resistor A cross-sectional area of resistor As shown in FIG. 1, the minimum resistivity occurs at 3.24 weight percent GeO based on the total weight of Fe O and GeO where x is 0.05 in Fe ,Ge,O Further additions of GeO increase the resistivity of these ceramic semiconductors. Presumably 3.24% GeO represents the maximum concentration of GeO that can be incorporated into the alpha-Fe O lattice. Similar data were obtained for comparative purposes with the Fe O /TiO system.

The data of Table I were used to prepare FIG. 1, a semilogarithmic plot of resistivity as a function of concentration GeO (in weight percent), when formed at different temperatures. FIG. 2 is a similar plot of data obtained similarly on reacting TiO with Fe o From the Figures, the relative temperature insensitivity of the Fe O /GeO system versus the Fe O /TiO system is apparent, at similar processing temperatures.

Table II compares the resistivities and resistivity ratios of fired Fe O /GeO and Fe O TiO bodies at the same weight percent concentration. These data again demonstrate TABLE II RESISTIVITY AND RESISTIVITY RATIOS FOR Fe O /GeO- AND Fe O /TiO AS A FUNCTION OF PROCESSING TEMPERATURE (48 HR. F!R1NG TIME) x in Resistivity. 25C. (ohm-cm.) Ratio: Ratio: Oxides Fe ,Me,.O 1000C. 1 100C. 1200C. Resist. (1000C.) Resist. (1000C.) (wt./r) Resist. (1100C.) Resist. (1200C.)

99.5% Fe o 0.008 40 39.7 31.5 1.0 1.3

0.5% GeO 99.5% Fe O 0.010 1.300 52 11.3 25 115 0.5% TiO the relative insensitivity of fired Fe O /Geo ceramic semiconductor bodies to processing temperatures when compared to Fe O- ;/TiO bodies.

Table III shows the NTCR characteristics of selected compositions of the invention. The coefficient of resistance is expressed as a fractional change in resistance/C. and commonly is referred to as a. a was determined from the following relationship:

it; L a HR dT 12 where 1 B slope of the linear plot 111 R vs. 70K

3. An electrically conducting shaped ceramic body, having a negative temperature coefficient of resistance, comprising the oxide of claim 1.

4. An electrically conducting shaped ceramic body, having a negative temperature coefficient of resistance, comprising the oxide of claim 2.

5. An electrically conducting shaped ceramic body according to claim 3, additionally comprising unreacted GeO wherein the total GeO present, in both the semiconductive oxide Fe Ge,,.O and as unreacted GeO is up to about 10% by weight of the total weight of Fe Ge O and GeO present.

6. An electrically conducting shaped ceramic body according to claim 4, additionally comprising unreacted GeO wherein the total GeO present, in both the semiconductive oxide Fe ,Ge O and as unreacted 6e0 is up to about 10% by weight of the total weight of Fe Ge O and GeO present.

7. An electrically conducting shaped ceramic body according to claim 3, additionally comprising unreacted GeO wherein the total GeO present, in both the semiconductive oxide Fe ,Ge,O and as unreacted GeO is up to about 5% by weight of the total weight of Fe Ge O and GeO present.

See footnote Table 1.

I claim: l. Semiconductive oxides of the hexagonal alpha- Fe O crystal structure having the formula Fe ,Ge,O wherein x is in the range 0.0001-005.

2. Oxides according to claim I wherein x is about 8. An electrically conducting shaped ceramic body according to claim 4, additionally comprising unreacted GeO wherein the total GeO present, in both the semiconductive oxide Fe ,Ge,O and as unreacted 6e0 is up to about 5% by weight of the total weight of Fe Ge O and GeO present. 

1. SEMICONDUCTIVE OXIDES OF THE HEXAGONAL ALPHA-FE-203 CRYSTALS STRUCTURE HAVING THE FORMULA FE2-IGEI03, WHEREIN X IS IN THE RANGE 0.0001-0.05.
 2. Oxides according to claim 1 wherein x is about 0.05.
 3. An electrically conducting shaped ceramic body, having a negative temperature coefficient of resistance, comprising the oxide of claim
 1. 4. An electrically conducting shaped ceramic body, having a negative temperature coefficient of resistance, comprising the oxide of claim
 2. 5. An electrically conducting shaped ceramic body according to claim 3, additionally comprising unreacted GeO2, wherein the total GeO2 present, in both the semiconductive oxide Fe2 xGexO3 and as unreacted GeO2, is up to about 10% by weight of the total weight of Fe2 xGexO3 and GeO2 present.
 6. An electrically conducting shaped ceramic body according to claim 4, additionally comprising unreacted GeO2, wherein the total GeO2 present, in both the semiconductive oxide Fe2 xGexO3 and as unreacted GeO2, is up to about 10% by weight of the total weight of Fe2 xGexO3 and GeO2 present.
 7. An electrically conducting shaped ceramic body according to claim 3, additionally comprising unreacted GeO2, wherein the total GeO2 present, in both the semiconductive oxide Fe2 xGexO3 and as unreacted GeO2, is up to about 5% by weight of the total weight of Fe2 xGexO3 and GeO2 present.
 8. An electrically conducting shaped ceramic body according to claim 4, additionally comprising unreacted GeO2, wherein the total GeO2 present, in both the semiconductive oxide Fe2 xGexO3 and as unreacted GeO2, is up to about 5% by weight of the total weight of Fe2 xGexO3 and GeO2 present. 